A problem faced by the prior art was measuring the hydraulic head within an electric furnace in which dry particulate batch material is directly melted, wherein the liquid level of the furnace could not be sensed, and wherein the level of the solid-to-liquid transition state could not be determined nor its effect on the hydraulic head predicted.
The prior art solution to this problem was to inject bubbles through the bottom of the furnace. The entrance pressure of the bubbles was sensed and this pressure was then used as an indication of the hydraulic head within the furnace.
However, this method of measuring the hydraulic head has the obvious disadvantage of introducing extraneous material into the furnace, as well as cooling the furnace and disturbing the furnace process.
A co-pending application Ser. No. 864,429, filed Dec. 27, 1977, and assigned to the common assignee, discloses an ultrasonic batch level sensor to regulate the batch thickness.